Polyether polyols, useful intermediates for the production of polyurethanes, are manufactured commercially by polymerizing propylene oxide in the presence of hydroxyl or amino group-containing initiators and basic catalysts. While the nominal functionality of the polyol product is that of the initiator, the average hydroxyl functionality is actually less than the nominal functionality. The lower actual functionality results because some of the propylene oxide isomerizes to allyl alcohol under the basic reaction conditions, and the allyl alcohol is propoxylated to give a polyether monol impurity. Polyether monol impurities, which are estimated by measuring polyol unsaturation, are preferably minimized because they adversely impact polyurethane physical properties.
The isomerization side reaction limits the conventional polyether polyol synthesis in two important ways. First, the reaction must be performed at temperatures within the range of about 95.degree.-120.degree. C. to achieve acceptable reaction rates and to minimize propylene oxide isomerization. At higher temperatures, polymerization rates are faster and batch times are shorter, but the isomerization reaction rate also increases so that the the unsaturation level of the polyol exceeds acceptable limits. At lower batch temperatures, the batch times are measured in days and weeks instead of hours. Thus, productivity is limited because higher reaction temperatures cannot be used without sacrificing polyol quality. Second, polyols having equivalent weights greater than about 2,000 cannot generally be made using conventional base catalysts without generating excessively high levels of monol impurities.
Previous approaches to preparing polyether polyols that contain reduced levels of polyether monol impurities have focused on preparing polyols at lower temperatures with conventional catalysts, or have used special catalysts such as double metal cyanide compounds. Because low-unsaturation polyols can give improved polyurethanes, synthetic routes to the polyols are of interest.
Propenyl unsaturation is preferred over allylic unsaturation because propenyl groups are readily converted to hydroxyl end-groups by acid hydrolysis or ion-exchange treatment. Polyether polyols made in a conventional process have predominantly allylic unsaturation. The proportion of propenyl unsaturation can be increased marginally by increasing the reaction temperature, but then the total unsaturation far exceeds acceptable limits. Therefore, it is not practical to raise reaction temperature in a conventional process to obtain polyols with a higher proportion of propenyl end-groups.
A preferred process would allow preparation of polyether polyols at relatively high temperatures (130.degree.-180.degree. C.) with conventional base catalysts, but without generating excessive levels of monofunctional polyether impurities. Preferably, the process would permit fast epoxide feed rates, short batch times, and reduced catalyst requirements. A preferred process could be performed at low epoxide concentrations to improve process safety. A process that could use a conventional base catalyst to make low-unsaturation, high equivalent weight polyols would be valuable. A preferred process would enable the preparation of polyols having relatively low total unsaturation, with a relatively high proportion of the unsaturation being derived from propenyl end-groups.